<template>
    <ul class="list">
       <li 
         class="item"
         v-for="item of letters"
         :key="item"
         :ref="item"
         @touchstart.prevent="handleTouchStart"
         @touchmove="handleTouchMove"
         @touchend="handleTouchEnd"
         @click="handleLetterClick"
        >
        {{item}}
        </li>
       
    </ul>
</template>

<script>
export default {
    name: 'CityAlphabet',
    props: {
        cities: Object
    },
    //计算属性
    computed: {
    letters () {
      const letters = []
      for (let i in this.cities) {
        letters.push(i)
      }
      return letters
    }
   },
    data () {
        return {
           touchStatus: false,
           startY: 0,
           timer: null
        }
    },
    updated () {
        this.startY = this.$refs['A'][0].offsetTop
    },
    methods: {
        handleLetterClick (e) {
            //console.log(e.target.innerText);
            //向外出发事件,实现兄弟组件之间的传值
            this.$emit('change',e.target.innerText)
        },
        handleTouchStart () {
            this.touchStatus = true
        },
        handleTouchMove (e) {
            if (this.touchStatus) {
                if(this.timer){
                    clearTimeout(this.timer)
                }
                this.timer = setTimeout(() => {
                    const touchY = e.touches[0].clientY - 74
                    const index = Math.floor((touchY - this.startY) / 20)
                    if(index >= 0 && index < this.letters.length){
                        this.$emit('change',this.letters[index])
                    }
                }, 16)
            }
        },
        handleTouchEnd () {
             this.touchStatus = false
        }
    }
}
</script>

<style lang="stylus" scoped>
    @import '~styles/varibles.styl'
    .list
        display flex
        flex-direction column
        justify-content center
        position absolute
        top 1.8rem
        right 0
        bottom 0
        width .4rem
        .item
            line-height .4rem
            text-align center
            color $bgColor

</style>

